This invention relates generally to force control of mechanical devices. More specifically, it relates to a very accurate, high bandwidth torque actuator with a built-in feedback reaction sensor.
Force control is often important in applications where a machine or robot has a member or members that interact with the external world. These machines include numerically controlled machine tools, mechanical arms and hands that assemble, paint, or handle parts and materials, and teleoperated or autonomous robots that can perform tasks in factories, at the ocean bottom, or in outer space. The world in which the robot or machine acts may be well defined, or unknown. Many robotic tasks in assembly or pick and place manipulation can require a control of the force exerted on an object so as not to damage it or the robotic device. In unknown environments it is important to have excellent force control to sense and react to collisions. Fast reaction requires a high bandwidth. Increasingly the capabilities of robots are becoming limited by the accuracy and bandwidth of the force control.
Force control in robotics falls into one of three categories: passive, active, or a combination of passive and active. Remote Center Compliance devices, as described in a 1977 Ph.D. thesis at M.I.T. by S. H. Drake entitled "Using Compliance In Liew Of Sensing Feedback For Automatic Assembly", are representative of passive control. They use compliance to control and limit forces at the end-effector. In active control, commands to an actuator control the forces exerted by manipulators. Active force control methods include feedforward control and feedback control. Active feedforward systems are most effective when the environment is known and relatively static. Control of joint torques can be adjusted through servo gains or precomputed torques. The actuator then produces the output torque computed. In general, these systems are imprecise, require large amounts of processing time, and are not well suited to operation in changing, dynamic situations, or situations where the operating environment is not well known.
Force and torque sensors are known to provide feedback control. Typically feedback controls fall into one of four types--damping control, stiffness control, impedance control or hybrid position/force control. Damping control uses an inverse damping matrix of variable velocity feedback gains to transform forces into joint velocity commands. Stiffness control uses proportional gains to produce a linear relationship between position error and commanded force. In impedance control, damping and stiffness matrices are used to convert position, velocity and acceleration errors into actuator commands. In the hybrid arrangement, two control systems operate simultaneously, one to close a position loop and the other to close a force loop.
Placement of the sensors is also important in determining whether the force control will operate well. Studies have demonstrated that placing the force sensor on an end effector necessitates lower gains to avoid instability when the end effector is in contact with a stiff environment. See, for example, "Force Feedback Control Of Fine Manipulator Motions" by D. E. Whitney, in the Journal of Dynamic Systems, Measurement and Control, Vol. 99, No. 2, (June 1977), pp. 91-97. Other research has shown that dynamics between a sensor and an actuator add poles to the system that often lead to instabilities. These instabilities were overcome if the sensor and actuator were colocated, or if one closed joint torque loops rather than end point force loops. Studies have also demonstrated a positive effect on bandwidth using joint torque sensors. Direct analog feedback of this type avoids calculations associated with Jacobian transforms needed in other active feedback control systems and thereby decreases the response time. However, colocation will not ordinarily provide a precise measure of the output torque at the end-effector.
Even with feedback and colocation, known arrangements for coupling the sensor into the drive have drawbacks. In one arrangement the sensor is connected in line with a motor output shaft. Slip rings or some other arrangement must connect to the shaft, a transmission member, or the output joint to obtain information as to the torque output. This arrangement interferes with the power and signal transmission. Also the sensor bandwidth is limited by the bandwidth of the transmission, which is typically low, e.g., less than 50 Hz. Torque tables are also known where the actuator is compliantly coupled to the torque table and strain gauges measure the reaction torque as a function of the angular displacement of the actuator in response to the reaction torque. The mass of the actuator limits bandwidth.
U.S. Pat. No. 4,384,493 to Grunbaum discloses a device that measures the output torque of a motor shaft by measuring the reaction torque exhibited by a rotation of the motor housing with respect to a fixed motor mounting flange. Strain gauges measure the torque produced by this rotation through the flexure of a bolt fixed on the motor and engaged at one end in a recess on the mounting flange. While this arrangement overcomes some of the problems with the prior art feedback actuators noted above, it is limited to an arrangement where a flange mounts the motor at one end. It also supports the shaft at one bearing and is therefore susceptible to axial and radial moments applied to the shaft. This arrangement is therefore not capable of extreme accuracy and is limited in its applications.
Whole arm manipulators (WAM's) place extreme demands on the force control system since the manipulator must sense and control forces anywhere along a link precisely, accurately, and rapidly. The WAM described in U.S. Pat. No. 5,046,375 has certain inherent force control advantages. It uses a highly efficient cable transmission that exhibits almost no friction or backlash, is backdrivable, lightweight, and has a high aspect ratio. This combination of advantages has shifted the limits on force accuracy and bandwidth of the system from the transmissions to the actuators driving the transmissions.
In theory, the torque output of an electric motor is linearly related to the current flowing to a motor, and therefore measuring the current should give an accurate measure of the torque. However, this approach does not take into account non-linear effects such as friction, (Coulomb, static, and viscous damping), torque ripple and dynamic effects. These non-linear effects can introduce errors as large as 10%. For high accuracy applications such as WAM, accurate feedback of the actual system output torque is required.
In practice, even with actual feedback from joint sensors, torque ripple, friction, sensor-actuator dynamics and other factors limit closed loop torque accuracy to about 8% of full output. Feedforward ripple compensation can usually reduce this error to about 5%, which is acceptable for many applications, but not for others such as precise contact detection. Low error is important whenever high accuracy or a high dynamic range is required. Dynamic range is defined as the ratio relating maximum output torque to the torque precision. Dynamic range allows a comparison, for example, of small actuators with a highly accurate torque output to actuators exhibiting a wider operating range, but lower accuracy.
Despite the known desirability of good control on the torque output of an actuator in robotics and other applications, no known arrangements provide the high accuracy, large bandwidth, and mounting versatility which is becoming necessary for further advances in many robotics applications such as whole arm manipulation.
It is therefore a principal object of this invention to provide a torque actuator with a very high degree of accuracy and precision and with a high bandwidth.
Another principal object is to provide such a high accuracy, responsive actuator which is compact, lightweight and can be mounted in a system in a wide variety of locations and orientations.
Still another principal object is to provide an actuator with the foregoing advantages which is substantially insensitive to torque ripple, friction, and the dynamics of the system.
Still another advantage is that the actuator can be sealed to allow underwater or space operation while still providing all of the foregoing advantages.
Yet another advantage is that the actuator can circulate a cooling liquid over its windings to allow operation at higher maximum output torques than otherwise attainable.
Another object is to provide a torque actuator with all of these advantages which is easily assembled and uses many standard components, and therefore has a favorable cost of manufacture.